The functionalitydoes the following:
1. Provide options for you to choose which users you want to contact: Newsletter Block Subscribers, Customers, and Customers who Subscribed to Newsletter.
2. You can check all of them and it will remove any duplicate emails before it sends the newsletter, so that people don’t receive two.
NOTE: Newsletter Block Subscribers don’t have First or Last Names, so the FIRSTNAME token will be ‘Subscriber’ and LASTNAME will be blank in their case.
Here is a Video-Tutorial: http://www.prestashopbr.com/tutoriais/newsletter/